The present invention relates to electronic ballasts for discharge lamps and lamp fixtures.
Electronic ballasts with dimming functionality are well known in the art. (See, e.g. Japanese Unexamined Patent Publication No. 2007-172933). FIG. 6 is a circuit diagram of a conventional electronic ballast. In this example, the ballast includes a half-bridge inverter circuit 1 having two switching elements Q1 and Q2, wherein a series circuit including the switching elements Q1 and Q2 is connected between both ends of a DC power source Vdc. A series resonant circuit 2 including an inductor T1 and a capacitor C1 is connected between a connection point of the switching elements Q1 and Q2 and a ground GND of the DC power source Vdc. The ballast is coupled to a discharge lamp FL across resonant capacitor C1 and through capacitor C2 which is used for resonant and DC blocking. Lamp filament F1 is connected to a preheating circuit 3 having a series circuit including an inductor L1 and a capacitor C3 as well as a preheating source n1. A second lamp filament F2 is connected to a second preheating circuit 3 also having a series circuit including an inductor L2 and a capacitor C4 as well as a preheating source n2. Preferably preheating sources n1 and n2 are set to have a same operation frequency.
In this conventional embodiment, a dimming signal inputted from dimming control 8 causes a frequency control circuit 5 to determine an operation frequency in the switching elements Q1 and Q2. The switching elements Q1 and Q2 are turned on/off alternately by a driving circuit 6 using a determined operation frequency. The switching elements Q1 and Q2 are turned on/off alternately to convert a DC voltage from the DC power source Vdc into a high-frequency voltage. An alternating current is made to flow in the discharge lamp FL to operate the discharge lamp FL at high frequency. The resonant circuit 2 including the inductor T1 and the capacitors C1 and C2 is connected to a power supply path to the discharge lamp FL. Energy supplied to the discharge lamp FL can be adjusted by a relationship between an operation frequency of the switching elements Q1 and Q2 and a resonant frequency of the resonant circuit 2.
A DC component detector 7 is connected in parallel to the discharge lamp FL which outputs to voltage comparator EL an output signal corresponding to a positive or negative DC voltage component generated in the discharge lamp FL. The inverter circuit 1 continuously operates in the case where the voltage comparator EL outputs a low signal. The output from the inverter circuit 1 is reduced or stopped by controlling an operation frequency in the switching elements Q1 and Q2 in the case where the voltage comparator EL outputs a high signal.
If one filament F1 or F2 is brought into a half-wave discharge state (commonly called as an emission-less state) due to consumption of an emitter (i.e. electron emissive material) at the end of the life of the discharge lamp FL, a DC voltage component is generated in the discharge lamp FL. In this case, the DC component detector 7 outputs an output signal corresponding to the DC voltage component. A signal from the DC component detector 7 is coupled to the voltage comparator EL which outputs a high signal in the case where the value of the inputted signal exceeds a reference voltage value Vref. This reduces or stops an output from the inverter circuit 1 for the purpose of lamp end of life circuit protection.
In such conventional end of life circuits, a resonant frequency in the preheating circuit 3 varies due to variations in the inductors L1 and L2 and the capacitors C3 and C4, even if the preheating sources n1 and n2 have the same operation frequency. This causes a phase difference in the constant preheating currents in the filaments F1 and F2 in accordance with changing a lamp dimming level, and creating a “hot spot” gap in the filaments F1 and F2 as a result. A hot spot position gap generated in the filaments F1 and F2 then causes a DC voltage component in a high-frequency voltage generated in the discharge lamp FL. Accordingly, a lamp end of life condition may be erroneously detected due to the DC voltage component, leaving a possibility that a protection function may be activated.
The prior art often uses the following method to avoid a false end of life detection and shutdown malfunction. When a dimming signal for changing the inverter operation frequency is inputted from the dimming control 8 to the frequency control circuit 5, a dimming signal detection circuit 9 detects a change in the dimming signal and outputs to timer 11 a signal corresponding to change in the dimming signal. In response to a signal inputted from the dimming signal detection circuit 9, the timer 11 outputs an ON signal to a driving circuit 10 so as to turn on a switch SW1 (such as transistor for example) for a predetermined period of time. Turning on the switch SW1 causes a signal from the DC component detector 7 to be fixed to a low level for a predetermined period of time.
FIG. 7 shows timing charts according to the conventional method, wherein the switch SW1 is not turned on if a dimming level is unchanged, because the dimming signal detection circuit 9 does not detect a change in the dimming signal. Therefore, a signal from the DC component detector 7 is inputted to the voltage comparator EL without making any changes, thereby allowing circuit protection for a discharge lamp FL whose life is at the end stage.
In contrast, in the case where a dimming level is rapidly changed, a DC voltage component in the discharge lamp FL is possibly inputted to the voltage comparator EL after the dimming signal stops changing, depending on a time constant of the DC component detector 7. Even in the case where a DC voltage component in the discharge lamp FL is inputted with a delay relative to a change in dimming signal, the timer 11 outputs the ON signal to the driving circuit 10 as long as the timer circuit 11 is set to have delay time which is sufficiently longer than a time constant of the DC component detector 7. This prevents an erroneous end of life detection and shutdown.
A DC voltage component occurring in changing a dimming level is observed in a rapid change in dimming level but is not observed in a gradual change thereof. In the case where a dimming level is changed as shown in the prior art (such as Japanese Unexamined Patent Publication No. 2007-172933, an operation to detect a lamp end of life condition is prohibited even when gradually changing a dimming level. This is problematic in a system with a constantly changing dimming level because the end of life end detection function may be inactive and circuit protection is impaired.